An optical module is a structure for optically coupling an optical element, such as a semiconductor laser element or a photodetector, with an optical fiber. The optical element should be protected against moisture from the viewpoints of stabilization of properties and high reliability. In general, these optical elements are housed in a case, and the case is hermetically sealed to protect the optical element against moisture outside the case.
Methods for hermetically sealing the case are roughly classified according to the structure of the optical module into two methods, that is, a method wherein a window permeable to light is provided in the case and an optical element provided within the case is optically coupled with an optical fiber outside the case through the window and a method wherein an optical element and an optical fiber are coupled with each other within the case. In recent years, the latter method is mainly adopted from the viewpoint of reducing the size of the optical device.
In a structure provided by the latter method, that is, in a structure wherein an optical element and an optical fiber are coupled with each other within the case, hermetical sealing should be performed between the optical fiber and the case.
A conventional hermetically sealed structure in an optical fiber introducing section is proposed, for example, in Japanese Patent Laid-Open No. 92334/1995. The hermetically sealed structure in an optical fiber introducing section described in this publication comprises: a case; a silicon substrate, with an optical element mounted thereon, fixed within the case; and an optical fiber disposed at a position where the optical fiber is optically coupled with the optical element, and fixed to the silicon substrate.
The optical fiber used in this case is a metal-coated fiber formed by coating a metal layer on the surface of an optical fiber strand, with a resin coating removed therefrom, by plating, sputtering or other method. The coating metal is a material having good solderability, such as nickel or gold. The metal-coated fiber is inserted into between the case and the cover, followed by soldering. As a result, the solder joins the case to the cover and, at the same time, joins the metal layer on the surface of the metal-coated fiber, thus hermetically sealing the interior of the case. Another conventional hermetically sealed structure in an optical fiber introducing section is described, for example, in Japanese Patent Laid-Open No. 198973/1995, wherein a low-melting glass is used.
In this hermetically sealed structure, a quartz optical fiber is provided without coating of a metal layer, and an optical element and the optical fiber are optically coupled with each other, followed by fixation of the optical fiber onto a silicon substrate.
The optical fiber is then inserted into between a case and a cover, a low-melting glass is provided in a portion, between the case and the cover, in its whole circumference, and the low-melting glass is then heat-melted to hermetically seal the interior of the case.
Still another conventional hermetically sealed structure in an optical fiber introducing section is such that the circumference of an optical fiber is sealed with a resin having a hermetically sealing property. In this hermetically sealed structure in an optical fiber introducing section, an optical fiber is provided without coating of a metal layer, an optical element and the optical fiber are optically coupled with each other, followed by fixation of the optical fiber onto a silicon substrate.
The optical fiber is then inserted into between a case and a cover, the resin is filled into a portion between the case and the cover, in its whole circumference, and the resin is then cured to hermetically seal the interior of the case.
The conventional hermetically sealed structures in an optical fiber introducing section have the following respective problems.
(1) The hermetically sealed structure in an optical fiber introducing section using a metal-coated fiber suffers from a problem that the cost of coating the metal layer onto the optical fiber is high.
An additional problem involved in this structure is that selective coating of the metal layer onto the optical fiber introducing section alone lowers the mass productivity.
Further, when the optical fiber is disposed on a V-groove in order to conduct positioning of the optical element and the optical fiber relative to each other, a variation in the thickness of the metal layer resulted in varied outer diameter of the optical fiber, disadvantageously deteriorating the positioning accuracy.
(2) For the hermetically sealed structure in an optical fiber introducing section using a low-melting glass, since the melting point of the low-melting glass is generally high and 430.degree. C. or above, at the time of sealing, the low-melting glass is likely to cause damage to the optical element fixed portion within the case and the optical fiber provided with a protective coating.
(3) The hermetically sealed structure in an optical fiber introducing section using a resin has problems including that the airtightness is greatly influenced by resin coating methods, seal path length, curing conditions and the like, making it impossible to stably conduct the sealing work, the hermetically sealing level of the resin per se is about 10.sup.-5 atm.cc/sec and unsatisfactory for use in optical modules, the reliability under severe environmental conditions such as high temperature and high humidity is low, and the sealed structure is not resistant to heat.